The effect of backrest angles on discomfort caused by fore-and-aft back vibration.

The effect on discomfort of the frequency and the backrest angle of x-axis (fore-and-aft) vibration of backrest have been studied. The method of adjustment was employed to obtain contours of equivalent comfort for three types of rigid backrests, whose surface were vertical, inclined 20 degrees and 40 degrees from vertical. Eighteen subjects were required to adjust test vibration over the frequency range 2-80 Hz to produce discomfort equivalent to that caused by 10 Hz 0.25 m/s2 r.m.s. sinusoidal reference vibration for each backrest. The direction of vibration was perpendicular to backrest surface. Results shows that the sensitivity at 20 and 40 Hz on inclined backrests is significantly about 1.4 to 1.5 times greater than that on a vertical backrest (p < 0.01) but there is no significant difference between the two contours for inclined backrests. The contour for a 'vertical' backrest obtained in this experiment agrees with the contour calculated from the frequency weighting factors for x-axis back vibration, Wc, in ISO2631-1, but the contours for inclined backrests do not agree with the contour well.